Although the target of the enters-the-battlefield ability must not be a land, lands with the same name as that permanent will be exiled.

10/1/2012

The enters-the-battlefield ability has only one target. The other permanents with that name aren't targeted. For example, a permanent with protection from white will be exiled if it has the same name as the target nonland permanent.

10/1/2012

If the target nonland permanent is an illegal target when the enters-the-battlefield ability tries to resolve, it will be countered and none of its effects will happen. No permanents will be exiled, including those with the same name as the target.

10/1/2012

If Detention Sphere leaves the battlefield before its enters-the-battlefield ability has resolved, its leaves-the-battlefield ability will trigger and do nothing. Then the enters-the-battlefield ability will resolve and exile the targeted nonland permanent and other permanents with that name indefinitely.